It is widely known that a soft handover, which performs a handover without changing a frequency, can be used in a code division multiple access (CDMA) cellular system because all base stations of one wireless communication service provider can use the same frequency.
The soft handover is a method for maintaining a communication link by simultaneously transmitting the signal to the source base station and the neighbor target base station without changing a communication frequency when a mobile station is located at boundary of cells, that is, the mobile station moves from a coverage of the source base station to another coverage of the neighbor base station, disconnecting a communication link to the source base station, if the signal intensity of the signal from the source base station is weaken below a reference signal intensity, and maintaining the communication link to the neighbor base station.
The above-mentioned soft handover provides a seamless handover by eliminating short disconnections of the signal, which is a chronic problem of an analog system, decreases a probability of call loss and maintains high quality communication.
However, the soft handover is not applicable in case the communication service provider allocates a different number of frequencies to each of the neighbor base stations according to a call intensity by considering an economy of a network design and an efficiency of investment, that is, the neighbor base station uses different frequencies from each other. In this case, if the mobile station using a specific frequency of the source base station moves to the cell of the neighbor base station, which does not use the specific frequency, then the soft handover cannot be applicable, therefore, the hardware handover has to be used.
FIG. 1 is a view illustrating a conventional wireless communication system. It shows a hard handover situation between two base stations.
Referring to FIG. 1, a target base station 130 does not support a communication frequency f2, which is used for communication between a mobile station 110 and a source base station 120. In this situation, if the mobile station 110 has a dual-mode receiver, the mobile station demodulates a downlink signal of the source base station 120 through the communication frequency f2, and simultaneously measures a signal intensity of a new frequency f1 and acquires a synchronization of a transmission signal of the target base station 130.
However, the above-mentioned dual-mode receiver needs an additional radio frequency (RF) hardware comparing to a single-mode receiver. Therefore, a complexity of the hardware of the mobile station is increased.
For overcoming the above-mentioned problems, a compressed mode is defined at an asynchronous W-CDMA (FDD: Frequency Division Duplex) standard (Release '99) of a 3rd generation partnership project (3GPP), released on September, 2000.
FIG. 2 is a diagram illustrating a conventional structure of a compressed mode transmission.
Referring to FIG. 2, in the 3GPP standard, a frame has a 10 msec length and 15 slots.
In a compressed frame 220, data transmission is not permitted during a transmission gap (TG) 230. Instead of permitting data transmission during the TG 230, a transmission power of the compressed frame outside the TG 230 region is kept higher than the transmission power of a normal frame 210, therefore, a possibility of frame error can be maintained identically to the normal frame 210.
By using the compressed mode in FIG. 2, a mobile station having the singe mode receiver can search the signal intensity of the new frequency f1 in case of the handover situation shown in FIG. 1. That is, during the TG 230, the mobile station disconnects the current communication frequency f2, changes to the new frequency f1, measures the signal intensity of the new frequency, and then after passing the TG 230 region, the mobile station returns to the frequency f2and demodulates the signal of the frequency f2 again.
In other words, the mobile station can monitor the new frequency f1 before completely disconnecting the current established communication link in case of the handover situation as shown in FIG. 1, the synchronization of the downlink signal transmitted from the target base station can be acquired by using a synchronization channel and common pilot channel of f1, therefore, the current call of the downlink is not disconnected even if the current established call is disconnect and the hard handover to the new frequency f1 is performed.
However, in case of uplink, the target base station cannot receive any signal before the hard handover execution. So the target base station should synchronize with the new frequency f1 after the mobile station completely disconnects the old frequency f2 and starts with new frequency f1.
In the above-mentioned situation, even if high quality searcher is used in the target base station, at least 1 frame should be spent for acquiring the synchronization of the uplink, therefore, transmission frames are disconnected shortly at the moment.
Also, since a round trip delay between the mobile station and the target base station is unknown due to inter base station asynchronous operation of the 3GPP W-CDMA(FDD) type, if a cell coverage of the base station is wider, then a search window size becomes very large so a time for acquiring the uplink synchronization of the target base station is spent more than several frames.
In the above-mentioned case, the discontinuation of several frames may happen and in worst case, the call drop may take place. Also, in this case, since an uplink power control cannot operate adequately, a capacity of the uplink of the target base station can be decreased badly.
In the 3GPP W-CDMA standard (release 99), the handover is possible only in case a frame offset, which is a difference of a system frame number (SFN) of the target base station and a connection frame number (CFN), is known to the radio network.
Before performing the handover, the mobile station measures the SFN of target base station and calculates the frame offset, and then, report the frame offset information to the radio network.
The above-mentioned operation does not have any problem in the soft handover between identical frequencies but produce a problem in case the hard handover is performed between different frequencies.
That is, the single mode receiver of the mobile station has to use the compressed mode for acquiring the SFN of the target base station, however, since transmission gap length is less than 1 frame, it is impossible to acquire the SFN in 3GPP's Release '99 standard.
In the above-mentioned case, since the mobile station should obtain the SFN after completely disconnecting the currently established frequency and connecting to the new frequency, at least 50 msec of an additional disconnection is generated.
Instead of using the compressed mode, there is another method introduced for performing the handover in case the situation shown in FIG. 1, by establishing the communication link between the mobile station and the source base station, measuring the signal intensity of the received signal from the target base station by generating a dummy frequency in the downlink according to the frequency f2 and performing the handover between the frequencies, if the handover condition is satisfied by the measured signal intensity.
However, the above-mentioned method also may increase a complexity of the base station due to a necessity of generation the dummy frequencies for all frequencies used in the system.